When I think about what parameters of a carrier could be controlled with a
modulating signal, there's amplitude, frequency, phase, and waveshape, and
there are numerous ways to modulate these parameters, both continuous and
discrete. An example of waveshape modulation would be pulsewidth modulation
(PWM). It is possible to encode information digitally by modulating any of
those four parameters...it doesn't have to be FM. With more sophistication,
you could modulate more than one parameter at a time to get more information
out of a given bandwidth.

Have a look at "modulation" in Wikipedia
(http://en.wikipedia.org/wiki/Modulation).

- Doug


wrote in message
oups.com...
i guess i was trying to find out how many ways signal can be modulated.
even if the signal is digital, isn't it still modulated in fm?